1. Field of the Invention
The present invention relates to an objective having a high numerical aperture (NA) to realize a large-capacity optical disk, an optical pickup with the objective, an optical disk writer-reader with the objective, and an optical disk reader with the objective.
2. Description of the Related Art
Conventional objectives for compact disks (CDs) have numerical apertures (NAs) of 0.45 to 0.5 and employ laser beams of about 780 nm in wavelength to read and write the CDs. Objectives for digital versatile disks (DVDs) have NAs of about 0.6 and employ laser beams of about 650 nm in wavelength to read and write the DVDs.
To handle high-capacity optical disks, now being developed are next-generation pickups with objectives that have high NAs and operate on short-wavelength beams.
The short wavelength beams may include a blue laser beam of about 400 nm in wavelength.
Examples of objectives having high NAs are reported in the following papers:
(A) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 978-979, M. Itonaga et al. xe2x80x9cOptical Disk System Using High-Numerical Aperture Single Objective and Blue LDxe2x80x9d
(B) Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 937-942, I. Ichimura et al. xe2x80x9cOptical Disk Recording Using a GaN Blue-Violet Laser Diodexe2x80x9d
The paper (A) reports a system employing a single lens having a numerical aperture of 0.7 and the paper (B) a system employing two lens groups having a numerical aperture of 0.85.
Higher numerical apertures result in lowering system margins. To cope with this problem, the systems reported in the above papers further thin a CD transmission layer of 1.2 mm and a DVD transmission layer of 0.6 mm. The paper (A) mentions a thickness of 0.12 mm and the paper (B) a thickness of 0.1 mm. Although the thickness of a transmission layer of an optical disk depends on system margins, it is preferable to be about 0.3 mm or thinner.
The two-lens-group system reported in the paper (B) realizes a greater numerical aperture than the system of the paper (A). The system of the paper (B), however, needs an assembling process of two lens groups, and therefore, is disadvantageous to mass production and increases costs.
According to the paper (B), the two-lens-group system involves a working distance of about 0.13 mm, which is shorter than about 1 mm of a single-lens system in a conventional DVD system. The short working distance increases a risk of colliding with an optical disk, thereby deteriorating the reliability of the system.
Next-generation optical disk systems are required to have single objectives having numerical apertures of 0.7 or above.
It is possible to design lenses having high numerical apertures. Shotaro Yoshida details a method of designing a double-sided aspherical lens having a high numerical aperture in xe2x80x9cStudy in Aspherical Aplanatic Lens with Particularly Large Aperture Ratioxe2x80x9d in Tohoku University Institute of Scientific Measurements Report, March, 1958.
Also, Japanese Patent Laid Open Publication 4-163510 discloses a single objective having a numerical aperture of about 0.6 to 0.8.
A lens having a high numerical aperture can be designed but is not always manufacturable. For actual manufacturing, a designed lens must secure a manufacturing tolerance. In addition, the designed lens must be less affected by wavelength variations or wavelength width of source light, to decrease chromatic aberration.
A critical manufacturing tolerance for a double-sided aspherical lens for an optical disk is a surface-to-surface eccentricity tolerance. In addition to keeping the eccentricity tolerance, the lens must simultaneously satisfy requirements for axial aberration related to perpendicular incident light and off-axis aberration related to oblique incident light.
It is nearly impossible for a lens having a numerical aperture of 0.75 or higher to simultaneously satisfy these requirements.
In a double-sided aspherical lens, off-axis aberration worsens in proportion to an increase in the numerical aperture of the lens even if no consideration is made on the manufacturing tolerance of the lens. If the manufacturing tolerance is considered, the off-axis aberration worsens because the manufacturing tolerance, i.e., the eccentricity tolerance of the lens is securable only by sacrificing the axial aberration and off-axis aberration of the lens.
Although the axial aberration of the lens does not greatly worsen with the consideration of the eccentricity tolerance, the off-axis aberration of the lens greatly worsens when the numerical aperture of the lens is higher than 0.6 and when the eccentricity tolerance is of the micrometer order.
Chromatic aberration is usually put behind the manufacturing tolerance. Namely, the manufacturing tolerance of a lens is considered at first, and then, the shape of the lens is improved as high as possible to minimize the chromatic aberration of the lens.
Many studies have been made on the shapes of double-sided aspherical lenses to improve lens performance. Some of the studies are disclosed in Japanese Patent Laid Open Publications 5-241069 and 4-163510.
The publication 4-163510 discloses a range of lens shapes to ensure good performance. This disclosure mentions nothing about the securing of eccentricity tolerance. A second embodiment of the disclosure explains a lens whose numerical aperture is greater than 0.75 (0.8 for a wavelength of 532 nm). This lens causes a large aberration even on a slight eccentricity. The disclosure mentions nothing about chromatic aberration.
The disclosures cover a wide range of specifications, and therefore, are insufficient to actually design a good lens.
The two-lens-group system mentioned above involves a short working distance, and therefore, greatly increases a risk of colliding with an optical disk when the lens groups employ a higher numerical aperture. Optical disks are generally made of plastic, which unavoidably involves warp. A CD involves a warp of about 0.6 mm and a DVD involves a warp of about 0.3 mm, which is a double improvement from the CD. No further improvement is expected in optical disk warp because the warp depends on disk material. The two-lens-group system has a working distance of 0.13 mm as mentioned above. This working distance may differ depending on lens design but must not be increased greater than 0.2 mm, to make a pickup that employs the two-lens-group system compact. With such a short working distance, the lens system will collide with an optical disk if focus servo runs off due to disturbance, vibration, or defects during a disk write or read operation.
There is another paper (C) Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 1804-1807, G. Hashimoto et al. xe2x80x9cMiniature Two-Axis Actuator for High-Data-Transfer-Rate Optical Storage System.xe2x80x9d The paper (C) discloses a compact system of two lens groups having a numerical aperture of 0.85 and a focal distance of 0.88 mm. This system may realize a compact actuator or pickup operating at high speed. The system, however, involves a very short working distance of 0.1 mm to increase a risk of collision.
An object of the present invention is to provide an objective (objective lens or lens) for an optical disk, made of a double-sided aspherical single lens (singlet) having a numerical aperture equal to or greater than 0.75 and capable of minimizing axial aberration, off-axis aberration, surface-to-surface eccentricity aberration, and chromatic aberration. Also provided are an optical pickup, an optical disk writer-reader, and an optical disk reader each employing the objective.
An aspect of the present invention provides an objective for an optical disk, herein the objective has first and second aspherical surfaces, a numerical aperture NA) of the objective is equal to or greater than 0.75 and a radius of curvature R1 of he vertex of the first surface is defined as follows:
(1xe2x88x92D)A less than R1 less than (1+D)A,
A=B/C,
B=0.85 f(nxe2x88x921) and
C=n (0.60866xe2x88x920.11 t/fxe2x88x920.1272 d/f)(0.83+0.2 NA)
where n is a refractive index of the lens, f is a focal length of the lens, t is a thickness along optical axis through the center of the lens, d is the thickness of a transmission layer of the optical disk, and D is a positive number of 0.05, desirably 0.04, more desirably 0.03.
Desirably, another aspect of the present invention provides an objective for an optical disk, wherein the objective has first and second aspherical surfaces, a numerical aperture (NA) of the objective is equal to of greater than 0.75 and an angle u1xe2x80x2 between a highest ray passing through the lens and an optical axis satisfies the following condition:
(1xe2x88x92D)K less than sin(u1xe2x80x2) less than (1+D)K and
K=(0.60866xe2x88x920.11xc2x7t/fxe2x88x920.1272xc2x7d/f)(0.83+0.2xc2x7NA)xc2x7NA/0.85
where f is a focal length of the lens, t is a thickness along optical axis through the center of the lens, d is the thickness of a transmission layer of the optical disk, and D is a positive number of 0.06, desirably 0.05, more desirably 0.04.
Desirably, the highest ray entering the first surface of the lens is parallel to the optical axis.
Desirably, the objective of any one of the above aspects has a manufacturable eccentricity tolerance between the first and second surfaces and minimizes off-axis aberration.
Desirably, still another aspect of the present invention provides an objective for an optical disk, wherein the objective has first and second aspherical surfaces, a numerical aperture (NA) of the objective is equal to or greater than 0.75 and an angle between a normal to the first surface at a point where a highest ray enters and an optical axis is smaller than a predetermined angle. Desirably, the predetermined angle is, for example, 57 degrees, desirably 56 degrees, more desirably 55 degrees.
Desirably, still another aspect of the present invention provides an objective for an optical disk, wherein the objective has first and second aspherical surfaces, numerical aperture (NA) of the objective is equal to or greater than 0.75 and an angle 0 between a normal to the first surface at a point where a highest ray enters and an optical axis satisfies the following condition:
xcex8 less than xcex1xe2x88x9247.3 (0.85xe2x88x92NA)(degrees)
where xcex1 is an angle of 57 degrees, desirably 56 degrees, more desirably 55 degrees.
Desirably, in the objective of any one of the above aspects, the thickness t along optical axis through center of the objective and focal length f satisfy the following condition:
t greater than (1+E)f
where E is a number equal to or greater than 0, desirably 0.1, more desirably 0.2.
Desirably, the objective of any one of the above aspects may have an image magnification of 0. Namely, the lens desirably focuses parallel rays if the lens involves no manufacturing errors and if a wavelength of source light agrees with a reference wavelength.
Desirably, the objective of any one of the above aspects may be designed for source light of 450 nm or shorter in wavelength.
Desirably, the objective of any one of the above aspects properly operates on an optical disk having a transmission layer thinner than 0.4 mm, i.e., thinner than a DVD""s or CD""s transmission layer.
Desirably, the focal length f of the objective of any one of the above aspects may be 10 mm or shorter, desirably 3.5 mm or shorter.
The size (diameter) xcfx86 of a light flux that enters the objective of any one of the above aspects depends on the numerical aperture NA and focal length f of the lens and is expressed as follows:
xe2x80x83xcfx86=2xc3x97NA xc3x97f
If the focal length is 10 mm and NA is 0.75, xcfx86=15 mm. This diameter is large relative to a light flux of about 5 mm or smaller employed by many optical pickups. It is desirable, therefore, that the focal length is shorter than 10 mm. If xcfx86 less than =5 mm and NA=0.75, then f=3.33 mm. It is more desirable that the focal length is shorter than 3.5 mm.
The focal length of the objective of any one of the above aspects is desirably 0 or greater, more desirably 0.2 mm or greater.
Desirably, a working distance of the objective depends on the thickness of an optical disk. Desirably, the thinner the optical disk, the larger the working distance. Desirably, if there is a very thin optical disk, a very small lens having a short focal length will sufficiently work on the disk with a very short working distance. Desirably, to work on a front-read optical disk, an objective with a focal length of 0.1 mm may be designed. In this case, a minimum focal length of the objective is desirably defined as f greater than 0. However, there is no means at present to manufacture such a very small lens. Desirably, a practical minimum for the focal length of an objective is f greater than 0.2 mm.
Desirably, an upper limit of the thickness t of the objective of any one of the above aspects is determined to make a working distance xe2x80x9cdwxe2x80x9d of the lens positive. Desirably, the working distance dw is defined as follows:
dw=fbxe2x88x92d/nxe2x80x2
where d is the thickness of an optical disk, nxe2x80x2 is a refractive index of the optical disk, and fb is defined as follows:
fb=f(1xe2x88x92t(nxe2x88x921)/n/R1)
where R1 is the radius of curvature of the first surface mentioned above.
Desirably, as the lens thickness increases, the working distance becomes shorter. Desirably, to provide a proper lens, a finite working distance must be secured. Namely, an upper limit of lens thickness desirably must be in a range where a finite working distance is securable. Desirably, this range is determined by the focal length and thickness of a lens and the thickness of an optical disk.
Desirably, the lens thickness may be in the range of 1.5 mm to 3.5 mm.
Desirably, the objective of any one of the above aspects is applicable to an optical pickup. The optical pickup employing the objective irradiates a track on an optical disk with a light flux focused by the objective, to write and read information signals to and from the optical disk. Desirably, the optical pickup may have an image magnification of 0.
Desirably, still another aspect of the present invention provides an optical pickup including the objective of any one of the above aspects, a laser source, and a photodetector.
Desirably, a working distance of the objective of the optical pickup dependens on the diameter of an optical disk irradiated with a laser beam emitted from the laser source and is defined as follows:
(working distance) greater than 0.005xc3x97(optical disk radius)
Desirably, still another aspect of the present invention provides an optical disk writer-reader including the optical pickup of the above aspect and a write-read unit to write and read information to and from an optical disk through the pickup.
Desirably, still another aspect of the present invention provides an optical disk reader including the optical pickup of the above aspect and a read unit to read information from an optical disk through the optical pickup.